The present invention generally relates to direct-sequence spread spectrum (DSSS) communications systems, and particularly relates to characterizing multipath propagation characteristics in DSSS receivers.
In wireless communications systems, successfully extracting transmitted information from a received signal oftentimes requires overcoming significant levels of interference. Multipath interference represents one type of received signal interference that can be particularly problematic in certain types of wireless communications systems. For example, wireless LANs are typically employed in indoor environments that commonly include partitioned walls, furniture, and multiple doorways, along with various metallic and non-metallic building features. In these environments, transmitted signals follow multiple transmission paths of differing lengths and attenuation. Consequently, a receiver in such an environment receives multiple, time-offset signals of differing signal strengths. These multiple versions of the same transmit signal are termed xe2x80x9cmultipath signals.xe2x80x9d
The effect of multipath signals on DSSS receiver performance depends upon the particulars of the communications system in question. For example, in certain types of DSSS communications systems, multipath signals can actually improve receiver signal-to-noise ratio. To understand this phenomenon, it is helpful to highlight a few basic aspects of DSSS communications. DSSS transmitters essentially multiply an information signal by a pseudo-noise (PN) signalxe2x80x94a repeating, pseudo-random digital sequence. Initially, the information signal is spread with the PN signal, and the resultant spread signal is multiplied with the RF carrier, creating a wide bandwidth transmit signal. In the general case, a receiver de-spreads the received signal by multiplying (mixing) the incoming signal with the same PN-spread carrier signal. The receiver""s output signal has a maximum magnitude when the PN-spread signal exactly matches the incoming. received signal. In DSSS systems, xe2x80x9cmatchingxe2x80x9d is evaluated based on correlating the incoming PN-sequenced signal with the receiver""s locally generated PN-sequenced signal.
The spreading code (PN code) used by the transmitter to spread the information signal significantly influences the effects of multipath signals on receiver performance. DSSS transmissions based on a single spreading code with good autocorrelation properties (or on a small set of orthogonal spreading codes) allow the receiver to selectively de-correlate individual signals within a multipath signal relatively free of interference from the other signals within the multipath signal. By adjusting the PN-sequence offset used to generate its local PN despreading signal, the receiver can time-align (code phase) its despreading circuitry with any one of the multipath signals it is receiving. If the spreading/despreading PN code has good autocorrelation and cross-correlation properties, the receiver can recover the transmitted data from any one of these multipath signals without undue interference. Of course, it may be preferable to use only the strongest multipath signal(s) for information recovery.
Indeed, conventional RAKE receivers used in Code-Division Multiple Access (CDMA) digital cellular telephone systems exploit the above situation. CDMA transmissions use a relatively long, fixed spreading code for a given receiver and transmitter pair, which results in very favorable auto-and cross-correlation characteristics. RAKE receivers are well known in the art of digital cellular receiver design. A RAKE receiver includes multiple, parallel xe2x80x9cRAKE fingers.xe2x80x9d Each RAKE finger can independently synchronize with and de-spread a received signal.
By synchronizing the multiple RAKE fingers to the strongest received multipath signals (those with the highest correlation values), the RAKE fingers"" lock on to the strongest multipath signals. Because of the excellent correlation properties of the CDMA spreading codes, each RAKE finger synchronizes with and de-spreads one of the multipath signals relatively free from interference associated with the other multipath signals. Thus, each RAKE finger de-spreads a relatively clean signal and this allows the overall RAKE receiver to coherently combine (with time/phase aligrnent) the signals to form a combined output signal that represents the addition of the multipath signals. Coherently combining the multipath signals allows the RAKE receiver to achieve an improvement in signal-to-noise ratio (SNR), essentially meaning that multipath signals can actually improve reception performance in certain types of spread spectrum systems.
Unfortunately, the characteristics of many other types of spread spectrum communications systems greatly complicate how a receiver deals with multipath signals. Some types of DSSS systems use spreading codes with poor correlation properties. The IEEE standard for high data-rate wireless LANs, known as 802.11b, is a primary example of such a system. Standard IEEE 802.11 transmissions use a single spreading code combined with binary phase-shift keying (BPSK) or quadrature phase-shift keying (QPSK) to transmit data at 1 or 2 Mbps, respectively. The 802.11b extensions provide higher data rates by defining 5.5 and 11 Mbps transmission rates. These higher data rates use a modulation format known as Complimentary Code Keying (CCK). 802.11b CCK-mode transmissions use multiple spreading codes, and the spreading codes change across symbols. While providing the ability to achieve high data transfer rates and still maintain compatibility with the standard 802.11 1 and 2 Mbps channelization scheme, CCK modulation does include the drawback of making it more difficult for receivers to cleanly despread individual multipath signals.
Indeed, due to the relatively poor correlation properties of the spreading codes used in 802.11b, the various multipath signals can interfere with each other and result in inter-symbol interference (ISI) at the receiver. Thus, in contrast to the CDMA digital cellular scenario, multipath signals can significantly degrade receiver performance in systems operating under 802.11b standards. Of course, multipath signals may be problematic in any type of DSSS system that uses less-than-ideal spreading codes, so the problem is not limited to wireless LAN applications. Multipath interference in DSSS systems arises from both inter-chip interference (ICI) and ISI. For the purposes of this disclosure the term ISI is understood to include both ICI and ISI. From the perspective of a DSSS receiver, each transmitted symbol results in the reception of multiple symbols arriving with relative time offsets from each other, due to the multiple signal propagation paths between receiver and transmitter. ISI, as used herein, describes multipath interference arising from these multiple received symbols and can include interference arising from multipath signal delay spreads exceeding one symbol period.
In DSSS systems where the spreading code(s) do not allow multipath signals to be individually despread without interference, RAKE receiver techniques are not applicable. The basis of RAKE receiver operation assumes that each RAKE finger can cleanly despread a selected multipath signal, which is subsequently combined with the output from other RAKE fingers to form an overall RAKE receiver output signal. If the output from the individual RAKE fingers is corrupted by multipath interference, the combined signal will be compromised and RAKE receiver performance suffers.
Channel equalization offers a potential opportunity for improving receiver performance in a multipath channel. Unfortunately, conventional channel equalization techniques are not suitable for DSSS transmissions due to complexity. For any radio frequency channel, the term xe2x80x9cchannel-coherent bandwidthxe2x80x9d describes the portion of a given channel""s available bandwidth where a relatively flat frequency response may be observed. Typically, only a small portion of a wideband DSSS channel may exhibit a flat frequency response. Consequently, existing equalizers exploiting conventional digital filtering techniques are inappropriate for compensating a wideband DSSS channel for multipath interference. This inappropriateness results from the sheer complexity associated with implementing and training a conventional digital filter having a finite number of filter taps and corresponding filter coefficients that is capable of compensating the received signal for the complex frequency response of a wideband DSSS radio channel.
Existing approaches to DSSS receiver design do not adequately address multipath interference in systems where individual multipath signals cannot be despread relatively free of interference. As noted, these types of systems are typically based on less-than-ideal spreading codes, with IEEE 802.11b representing an example of such systems. Without the ability to handle multipath interference, such systems cannot reliably operate in environments with significant multipath interference. Existing approaches, including the use of RAKE receivers or conventional channel equalizers are either inappropriate or impractical.
Effective handling of multipath signals, whether for the purpose of interference compensation, such as in 802.11b environments, or for the purpose of coherent multipath signal combination, such as in RAKE receiver operations, depends upon developing accurate estimates of propagation path characteristics for one or more of the secondary propagation path signals included in the received signal. Under many real world conditions, the delay spread among the individual propagation path signals comprising a received multipath signal exceeds one symbol time, meaning that, at any one instant in time, the various propagation path signals may represent different information values (symbol values), making it potentially difficult to relate one propagation path signal to another. Without this ability, only multipath signals with propagation path delay spreads less than a symbol time may be processed substantially free from interference.
Thus, there remains a need for a method and supporting apparatus for identifying and characterizing secondary signal propagation paths relative to a main signal propagation path that accommodates a wide range of propagation path delay spreads, including delay spreads that exceed one symbol time. With the ability to determine time offsets between main and secondary signals over a range of less than to more than one symbol time, a communications receiver can accurately characterize secondary signal propagation paths relative to a main signal propagation path in variety of environments, even those with severe multipath conditions. Such a characterization method would allow for compensation of a received multipath signal in a broad range of radio signal propagation environments, even those with severe multipath conditions, thus enhancing communications receiver performance. This method and supporting apparatus would be particularly valuable in any type of DSSS communications system that relies on spreading techniques that do not intrinsically provide multipath interference rejection, but would also be valuable in any DSSS communications system subject to multipath signal reception.
The present invention includes an apparatus, referred to as a training circuit, for characterizing one or more secondary signal propagation paths relative to a main signal propagation path. Preferably, the training circuit of the present invention is included in an associated communications receiver that provides it with signals received via the main signal propagation path concurrently with a signal received via one of the secondary signal propagation paths. The training circuit accumulates magnitude samples from the main and secondary signals and uses them to compute a relative magnitude for the secondary signal. The training circuit also includes circuit resources for differentially decoding symbol values (phase values) received via the main and secondary signals, and correlation circuitry for determining secondary-to-main signal time offset using the differentially decoded symbol information. Additional resources within the training circuit compute possible phase offsets for the secondary signal relative to the main signal based on current and delayed symbol values, and supporting logic to identify a one of these possible phase offsets as being the closest-to-actual phase offset based on the time offset information. Thus, the training circuit provides time offset, magnitude, and phase information for a secondary propagation path signal relative to a main propagation path signal. The associated communications receiver may use information developed by the training circuit to improve multipath signal reception performance or cancel selected multipath interference arising from one or more of the secondary propagation path signals.
By adopting the main path signal as the reference, an information symbol or transmitted data item received through the main path signal may be considered to have an arrival time of t0, a phase of 0, and a magnitude of 1. A selected secondary path signal may then be concurrently compared to the main path signal to determine relative magnitude, phase, and arrival time, thereby characterizing the secondary signal propagation path parameters with respect to the main signal propagation path. Indeed, in exemplary embodiments, the associated communications receiver provides the training circuit with sequences of magnitude and phase values from the main path signal and a selected secondary path signal for a period of time sufficient to allow characterization of the selected secondary path signal. The communications receiver may repeat this operation for one or more additional secondary path signals.
Preferably, the training circuit includes integrators that integrate several successive magnitude samples from the main path signal and the currently selected secondary path signal. A comparison function uses these integrated magnitudes to determine a ratio for the magnitude of the secondary path signal relative to the main path signal. Since the main path signal is, preferably, the strongest signal, this relative magnitude is a fractional value. Other embodiments of the training circuit may implement other techniques for accumulating magnitude samples, and the number of magnitude samples accumulated will vary with design and performance requirements.
There is a two-fold problem associated with determining the time and phase offset for the secondary signal relative to the main path signal: (1) the secondary signal has some unknown phase shift relative to the main signal; and (2) the secondary signal may be offset from the main signal by more or less than one symbol period, in either a leading or lagging fashion. To eliminate the secondary signal phase shift, the training circuit includes differential decoders for the main and secondary signals. Differentially decoded symbol values (phase values) are processed by correlation circuitry included in the training circuit. Correlation operations identify the correspondence between symbols in a sequence of symbols received via the main path and symbols in a sequence of symbols concurrently received via the secondary path. Corresponding main and secondary path symbols values will have a minimum phase difference and, thus, a maximum correlation. Once the symbol correspondence is known, the training circuit can infer the time offset between the main and secondary path signal.
Additional circuitry within the training circuit, disposed in advance of the aforementioned differential decoding circuitry, supports determining the phase offset or difference between the main and secondary signals. This circuitry computes the phase difference as the difference between current and delayed samples of the main and secondary signal symbol phase values. Since the secondary signal symbol phase values corresponding to the main signal symbol phase values may be offset by more or less than one symbol period, in either a leading or lagging fashion, the training circuit computes the phase difference between current and delayed main and secondary signal symbol phase values. The previously determined time offset value is used to select the appropriate phase difference value.